1. Field of the Invention
This invention lies in the field of heat transfer apparatus. More particularly it concerns improving the heat transfer in a convection section of a furnace, by the addition of metal strips which are heated by convection from the flowing combustion gases, and which radiate heat to the pipes carrying the fluid to be heated in the furnace.
2. Description of the Prior Art
In the process industries, where fluid raw materials are converted into more useful or more valuable products through application of heat energy in well known manners to the raw materials, structures which are called "heaters" are employed.
A heater is composed of a furnace or combustion volume; burners to inject fuel into the furnace, in calculated manners, for provision of a supply of heat energy, such as may be required, and a sinuous tubular passage through the burner-heated areas. The tubular passage is entirely within the heated volume and spaced from the furnace structure.
The tubular passage is for closed transport of the fluid to be heated through the furnace areas where, due to burner-produced heat energy, the fluid, as it flows through the tubular passage, receives heat energy through a process which is typically identified as heat transfer -- that is, the heat energy which is supplied to the furnace areas passes through the wall of the tubular passage to enter the flowing fluid. Such heat-energy transfer results in increase in the state of molecular motion with the flowing fluid, and since absolute temperature varies as the square of the average molecular motion, the temperature of the flowing fluid is caused to rise to a preferred or required level.
In such processing action, it is not possible to recover all of the burner-produced heat for transfer to the flowing fluid, since residual combustion gases, vented to atmosphere after all possible heat recovery, are at temperature which is hundreds of degrees above the temperature of fuel and air supplied for burning. This residual gas temperature (stack temperature) is useful in that, because of it, the stack or chimney can produce `draft` or less-than-atmospheric pressure inside the furnace. The less-than-atmospheric pressure inside the furnace causes a required quantity of air to be drawn into the furnace for mixture with the fuel to permit fuel burning or combustion to occur to a required degree. However, the heating of stack gases results in heat-energy loss such that efficiency of heat recovery (thermal efficiency) seldom exceeds 82% and is typically in the order of 75% or less.
Prior to the fuel/energy crisis, and when supplies of fuel were both plentiful and cheap, there was small concern if the thermal efficiency of a process heater was slightly greater than 70%, and many furnace/heater installations were designed for 70%+ efficiency (not 80%+). These, or many of these, are still in operation to the great distress of their operators because of excessive fuel requirement, when fuel cost has increased many times, and is no longer plentiful. For the average heater, fuel costs per year have increased by hundreds of thousands of dollars, and this amounts to precious fuel wastage.
Design factors of the furnace or heater determine the thermal efficiency to be expected, and 1975 design practice dictates stack temperature at 400.degree. F. rather than 800.degree. F. upward, for added conservation of 11% of fuel burned for a required service, or more. But this is for heaters now being built. For existing heaters there is reason to improve thermal efficiency, as is obvious, but at very great expense for alteration, and at the expense of serious loss of product because the heater cannot be operated during the time the heater is being modified which may take up to 7 days.
In the art of process heater design, and to provide reasonable nomenclature, the heater, per se, is considered as three separate sections. These are the "radiant" section, the "convection" section, and the "stack" section.
Nomenclature for the three sections is based on the service performed by separate sections. Heat transfer is either through radiant effects; through convective effects or both, while the stack vents combustion gases to atmosphere while providing draft for induction of combustion-supporting air, or maintenance of less-than-atmospheric pressure within the heater structure. Preponderance of heat transferred is in the radiant section (typically 80% of total heat transferred) with smaller quantities of heat (typically 20%) transferred in the convection section. This heat transfer relative state is due to two factors. The first is that, in reference to relative heat transfer per square foot of tubular heat transfer surface, the higher combustion chamber temperature, plus effects of radiant heat transfer, plus certain convection effect causes greatest heat transfer to occur in the "radiant" section area. Even if the "radiant" and "convection" heat transfer surface areas should be equal, a significant preponderance of heat transfer would occur in the "radiant" areas. The "radiant" areas are defined as the areas which can "see" the burner flames and radiant combustion chamber surfaces, where these surfaces are typically formed of refractory material. The "convection" areas are defined as areas of tubular heat transfer surfaces which cannot see the burner flames or radiant combustion chamber surfaces. In all areas, there is some heat transfer by both mechanisms, according to the relative emissivities and temperatures of the radiant heat sources.
Emissivity denotes ability to emit radiation of energy as heat. The emissivity of heated gases (due to the presence of binary molecules such as CO.sub.2, H.sub.2 O and SO.sub.2 --SO.sub.3) is quite small and is typically 0.05, while the emissivity of refractory surfaces can be considered, typically, as 0.80 or greater (16 times greater or more). Radiant heat is as infra-red emission which is predominantly absorbed by, but partially reflected from the tubular heat transfer surfaces, according to the absorptivity of the surfaces. Relative absorptivity of surfaces is according to Kirchoff's Law which teaches that ability to emit is equal to ability to absorb. Radiant heat transfer between bodies is according to the Stefan-Boltzmann Law (Perry's Chemical Engineers' Handbook).
Convective heat transfer is due to flow of fluids, where the quantity of heat energy transferred is proportional to flow mass-velocity and temperature difference, and in the case of process heaters, the fluid is heated combustion gases from which a significant portion of combustion heat has been removed.
Thus, the convection section becomes a part of process heaters as means for final recovery of combustion heat (an `economizer`) as the combustion gases are compelled to give up heat energy as they move toward final venting to atmosphere, and total loss of residual heat energy, but in many existing process heaters, the convection section is far from adequate for suitable heat recovery. As has been pointed out, very expensive and time-consuming means for added heat recovery are available for these heaters. But in many cases, because of either high cost or because of lost process time, there is great reluctance to apply these means to heaters, and as a result, the heater operation is both wasteful of fuel and more expensive.
Earlier delineation of heater nomenclature as the "radiant section" as the "convection section" is convenient but not accurate. Radiation of heat energy between two adjacent bodies immediately begins to occur when the temperature of one body exceeds the temperature of the adjacent body. Heat energy transferred is related to the fourth-powers of the absolute temperatures; it is also proportional to emissivity and absorptivity.